Fish Microbiology and Quality Assurance (2+2)

Unit 5 - Foodborne pathogens

5.6. Foodborne gastroenteritis caused by Listeria
Among Listeria species, Listeria monocytogenes is most important human pathogen causing the disease listeriosis and is of great concern to special risk groups. The susceptible groups include pregnant women and their fetuses, cancer patients and others undergoing immuno-suppressive therapy, as well as diabetics and cirrhotics and the elderly. Although the risk of contracting listeriosis is less for normal, healthy individuals, they may also contract the disease.

Characters

• Listeria monocytogenes is a facultative intracellular pathogen. The organism enters the body through the intestine and has a variable incubation period from 1 day to a month or longer. The ingested cells enter the body through ileal villi cells, subsequently taken up by macrophage cells in the bloodstream, multiply inside the host cell and released after bursting of macrophages and liberated cells infect other cells.
• Listeria is widely distributed in the environment, human beings and a variety of animals including seagulls. Most of these environmental strains are non-pathogenic. Listeria sp. other than L. monocytogenes appears to be more common in tropical areas. Though it has been isolated from a variety of seafoods including refrigerated and frozen crabmeat, reports of listeriosis involving seafood is rare.
• Listeria are relatively heat resistant but unable to surive in foods receiving adequate heat treatment. The presence of L. monocytogenes in cooked seafood is mainly due to cross-contamination of the product or under processing.
• Listeria are aerobic under most circumstances but can be facultatively anaerobic and thus grow well under reduced levels of oxygen in packaged products. Listeria can also survive and grow under refrigeration temperature and also survive freezing conditions.
• Contaminated food is increasingly recognized as an important vehicle of L.monocytogenes. Dairy products, salads and vegetables have been implicated in outbreaks of listeriosis. Frequent isolations from seafood as well as its ability to grow in in chilled smoked salmon at +4oC is an indication of possible involvement of seafoodsin the transmission of L. monocytogenes.

Listeria species involved in food poisoning

• Six species of Listeria are currently recognized, but only three species, L. monocytogenes, L. ivanovii and L seeligeri are associated with disease in humans and / or animals. However, human cases involving L. ivanovii and L. seeligeri are extremely rare.
• Listeria are commonly identified by serotyping. Types 1-7 are known, with Types 1/a, 1/b, and 4b predominating as both environmental and clinical isolates. The serotyping scheme is based on both somatic and flagellar antigens. Phage typing has also been employed as a method of further identifying isolated strains. L. monocytogenes is subdivided into 13 serovars on the basis of somatic (O) and flagellar (H) antigens.

Symptoms
Typical Listeria infections result in septicemia, meningitis and encephalitis, and enteritis. High mortality is reported among infected individuals. Mortality rate of 29% is noticed among patients in a New England outbreak involving fluid milk. This causes the transitory flu-like symptoms at the initial stage of infection with or without symptoms of stomach disturbances and diarrhea. The actual disease known as listeriosis occurs only after the attainment of severe form of septicemia, encephalitis, lesions, or meningitis. All of these forms of listeriosis is noticedamong individuals who are not immunocompetent.

Prevention of outbreak
To prevent risk due to L. monocytogenes, FDA in the US has recommended complete absence in ready-to-eat seafood products such as crabmeat or smoked fish. This zero-tolerance is also required for products which receive a listericidal treatment, and for products which have been directly implicated in a food borne outbreak. However this restriction does not apply to raw product that will be cooked before eating. Low number of L. monocytogenes is allowed in other types of products in which the organism is shown to die-off. But, numbers greater than 10 L. monocytogenes / g are likely to constitute a risk to human beings particularly predisposed persons (very old, very young or immuno-suppressed). Following proper GMP and factory hygiene is expected to maintain the level of L. monocytogenes contamination on fish products at very low level of less than 1-10/g.